Numerous methods and apparatus have been developed for transmitting messages in a secure manner between stations. Many of the methods and apparatus provide for coding of the message prior to transmission and for decoding of the message at its destination subsequent to transmission. Two such methods and apparatus are described, for example, in U.S. Pat. No. 3,711,645 issued Jan. 16, 1973, to Kurt Ehrat, entitled Method And Apparatus For Coding Messages, and in U.S. Pat. No. 3,956,615 issued May 11, 1976, to Thomas G. Anderson, William A. Boothroyd and Richard C. Frey, entitled Transaction Execution System With Secure Data Storage And Communications. A third method and apparatus is described, for example, in U.S. Pat. No. 3,938,091 filed Feb. 10, 1976, to Martin M. Atalla, entitled Personal Verification System.
A common feature of such apparatus is the use of fixed, predetermined encoding-decoding keys to encode and decode data, or the use of variable encoding keys transmitted with the encoded message to a destination station for processing (the variable encoding keys being supplied by the user, or generated by the apparatus from user-supplied information). At the destination station, the transmitted variable-key is utilized by a decoder of the apparatus to decode the encoded message.
Whereas the use of variable rather than fixed encoding-decoding keys would seem to provide a greater measure of security in message transmission than do fixed keys (especially variable keys supplied by a user or generated by the apparatus from user-supplied information), the practice of transmitting a variable key so that it can be used in the decoding process reduces the security of the operation because unauthorized users could conceivably gain access to the key during the transmission of the key. It would be desirable, therefore, in improving the security of such data transmission operations (especially bank transaction operations or the like which require not only secure transmission of data but also accurate identification of users) to use a technique which provides for secure transmission of data without requiring transmission of matching encoding-decoding keys or of user-identification information. Also, it would be desirable in certain applications to obviate the need for dedicated pairs of matched encoders/decoders in order to enhance the flexibility of the data-transfer system and to reduce the start-up and changeover time inherent in placing conventional paired modules in the proper locations. In addition, it would be desirable to encrypt messages between specific persons or entities via an intermediate or central station which can function in a manner similar to a data switch that ties numerous entities together.